Means for close placement of electrode plates in a thermionic converter



March 9, 1965 J, MAYNARD 3,173,032

MEANS FOR CLOSE PLACEMENT OF ELECTRODE PLATES IN A THERMIONIC CONVERTERFiled Sept. 14, 1959 2 SheetsSheet 1 ,MHmunm INVENTOR.

JOHN T. MYJMHD Oqttorne gs March 9, 1965 J. T. MAYNARD MEANS F OR CLOSEPLACEMENT OF ELECTRODE PLATE IN A THERMIONIC CONVERTER Filed Sept. 14,1959 INVENTOR. JUHN T. AMI [MED BY III/111111111111? OVi'forneqs UnitedStates Patent T saunas MEAN Milt QLUSE PLACEMENT @F ELECTRGDE PLATES TNA THERMIGNKC CUNVERTER John T. Maynard, West Allis, Wis, assignor to A.0.

Smith Corporation, Milwaukee, Win, a corporation of New York Filed Sept.14, 1959, Ser. No. 839,702 2 Claims. (Ci. Bald-4) This invention relatesto a thermionic converter, and more particularly to the use of uniform,finely divided insulating particles disposed between the electrodeplates of a thermionic converter element to effect a very close spacingof the plates which increases the electron flow to the anode, electronemission from the cathode due to reduction of space charge, and thermalefiiciency oi the device.

There have been numerous attempts in recent years to develop efiicientheat-toelectricity converters. Various devices have been created whichutilize the phenomena of thermionic emission. The British Patent741,058, to David Malcolm .lohnstone, covers such a device. Basically,thermionic emission is the fiow of electrons from a conductor surfacesubjected to heat. When an electron absorbing or attracting surface, oranode, is placed in close proximity to an electron emitting metalsurface, and a complete external circuit is provided, the flow ofelectrons creates an electrical current through the circuit.

Other devices which convert heat to electricity are known asthermoelectric couples. These may be used in series to increase theavailable electromotive force, and are then referred to as thermopiles.Thermopiles and, more specifically thermoelectric couples, are comprisedof dissimilar metals or dissimilar (p and n type) semiconductorelements, which are physically connected at one end to form anelectrical junction. The open ends of the elements are electricallyconnected to a resistance load which consumes the generated power.Applying heat to the junction so formed, creates a'temperaturedifference and thermal gradient along each element, giving rise toSeebeck and Thomson potentials. The Seebeck potential is anelectromotive force'developed if two different homogeneous conductorphases are joined at both ends, and the two junctions are kept atdiiferent temperatures. The Thomson potential refers to the thermalgradient withina homogeneous conductor, and since it occurs in a singlematerial it is difiicult to measure.

The Seebeck and Thomsonpotentials created by heating the elementsproduce a source of electrical energy to power an external load. Thehigher the temperature differential that can be maintained between thehot and cold junctiornthe higher will be the electromotive forcepotential that is available to power an external load. Heat conductedfrom the hot junction through each element to the cold junction limitsthe maximum temperature differential that can be maintained, and hencethe available power output and thermal efiiciency is also limited.

In an GlTOl'i to reduce thermal conductivity, semicorb ductor elementsare doped with impurity atoms, usually resulting in' an undesiredincrease in element resistivity. Due to the relatively high resistivityand high thermal conductivity of the elements, the output currentdensity and voltage output per couple is inherently low.

. Athermionic generator is another thermoelectric device which issimilar to the thermo-couple. The thermionic generator has a portion ofthe thermo-couple circuit replaced with a separated'conducting' spacewhich may contain an inert gas, an ionized gas, or which may beevacuated. This minute, evacuated or gas filled space serves as anexcellent thermal barrier, preventing heat transfer from cathode toanode, while offering very little impedance to the flow of electrons.Radiation losses 3,173,032 Patented Mar. .9, 1965 from the hot cathodeare practically independent of the anode position or gap spacing.Extremely high tempera ture difiierences, or gradients, between theanode and the cathode may be easily maintained and are limited only bythe melting points of the cathode, and not by conduction losses, as inthe case of a semi-conductor thermo-couple.

The importance of a high cathode temperature is exhibited by theRichardson-Dushman thermionic emission equation for saturation currentunder zero field conditions. This equation must be modified to includeany additional potential fields to which an escaping electron may besubjected. One such force is a retarding space charge field above thecathode. Assuming that the anode temperature is sufiiciently low toprevent any current flow from the anode to cathode, thefollowing-equation illustrates thetemperature dependence of electronflow:

o 0 (c+sc) HCT Where:

T equals electron current density escaping to the anode, A equalscharacteristic saturation emission constant of the emitting surface, Tequals cathode emitter temperature, it equals Boltzmanns constant 5equals cathode work function potential (electron volts), p equalslspacecharge potential due to electron-electron interactions and collisionscattering.

It can be seen that any appreciable increase in the cathodeoperatingtemperature will greatly increase the output. current, andhence the overall efliciency of the thermionic converter. Morespecifically, increasing the cathode temperature has the desired efiectof increasing the amplitude of atom vibration and the number of elasticcollisions between electrons and atoms, thus increasing the number ofelectrons which have enough energy to escape from the surface potentialbarrier of the emitting metal. Once the electrons escape, they aresubject to whatever potential fields exist above the cathode surface.Electrons which .cscapeboth the potential barrier of the cathode and theretarding space charge potential field, are attracted to the anode dueto image charge. In order to trap the maximumnumber of the freeelectrons at the anode, it is desirableto place the anode as close inproximity as possible to the.cathode, leav.ing only sufficient spacebetween the cathode and anode plates to prevent electrical shorting.Closer spacing allows a higher current, higher power output and betteroverall thermal eiiiciency, although output voltage-may be slightlylower. Current and power increase approximately as a function of 1/ X2where x is the spacing, while the output voltage decreases, as only asmall linear function of the gap spacing as it is reduced.

k In view ofthe foregoing, it can be seen that a thermionic device,having a minutely separated cathode and anode has greater capacity toperform eflicientlywhen compared to a thermoelectric couple of the samesize and materials. Also, more intense heat sources can be used to drivea thermionic converter. Solar energy, atomic energy, and hightemperature combustion energy, or a combination thereof, maybe used tocreate direct current electrical energy.

ln general, the thermionic converter of thepresent invention comprises.an electron emitting cathode, an electron collecting anode disposed inintimate, spaced relationship to said cathode, and a plurality-of finelydivided particles of a substance having a high melting point, highresistance to crushing, and high electrical resistance at the workingtemperature disposed between the opposing surfaces of the anode and thecathode. A ceramic seal is employed around the edges of the anode andthe cathode for sealing the chamber therebetween and for electricallyseparating the members. Electrical conducting leads connect the anodeand cathode to a resistance load, thereby utilizing the electrical powergenerated by thermionic emission.

According to the invention, the minute particles, having a size in therange of .2 to 1000 microns, are formed of a substance such as diamond,metal oxides and the like and are applied to either the cathode or theanode surface by spraying, sputtering, printing, chemical coatingprocess, or electrode deposition.

The thermionic converter has no moving parts, utilizes high temperatureheat sources, and produces electricity directly from heat energy atimproved efiiciency because of the novel cathode and anode spacingmeans. Other objects and advantages of this invention will appear in thecourse of the following description.

The drawings illustrate the best mode presently contemplated forcarrying out the invention.

In the drawings:

FIGURE 1 is a vertical section of a thermionic converter with a coolingjacket surrounding the anode plate;

FIG. 2 is a top view of the device shown in FIGURE 1, with therespective layers broken away to show the random distribution of theparticles and the relationship of the cooling jacket to the anode plate;

FIG. 3 is a greatly enlarged sectional view showing the jagged particlesimbedded in the cathode and anode plate surfaces;

FIG. 4 is a sectional view of a modified form of the invention showing aseries of electrode plates in thermal cascade;

FIG. 5 is a bottom view of the thermal cascade device of FIG. 4; and

FIG. 6 is a perspective view with parts broken away to show internaldetails of construction of a second modified form of the inventionshowing a thermionic converter element in an atomic reactor.

FIGURES 1-3 illustrate a thermionic converter comprising a cathode plate1 and an anode plate 2 which are spaced apart by a plurality of minute,uniform size particles 3.

A cathode 1 is a thin, rectangular plate formed of a metal, a metallicoxide or a cermet having good thermionic emission properties, and whichwill remain in the solid state at the operating temperature range, whichmay be up to 4000 K. A cermet is a combination of a refractory metal anda highly emissive oxide and most of the cermet have high melting pointsand can be used as a cathode material at the higher operationtemperature ranges up to about 3500 K.

A surface coating 4 may be added to the cathode 1 as shown in FIGURE 1.The surface coating 4 is a crystal complex of barium oxide and strontiumoxide (BrO-SrO) from one to several monolayers thick, and itsubstantially covers the top surface of the cathode 1. The surfacecoating 4 increases electron emission of the cathode 1.

An anode 2 is a thin, rectangular plate and complements the cathode 1 insize and shape. The anode 2 is disposed parallel to, and in closerelationship to the cathode 1, being spaced apart therefrom at adistance in the range from .2 to 1000 microns by particles 3. The anode2 is formed of a metal such as nickel, and serves to collect theelectrons emitted from the cathode 1.

The particles 3 are a substance having a high melting point, highresistance to crushing, and high electrical resistance. The meltingpoint of the particles 3 is in the range from 1000 K. to 4000 K. and theparticular operating temperature of the thermionic converter determinesthe material selected for use as particles 3, since it is desirable thatthe particles 3 have a melting point more than 200 K. above theoperating temperature to insure permanent spacing of the cathode 1 fromthe anode 2.

The particles 3 should have a thermal conductivity less than 1.25ca1.-cm./cm. C./sec. Assuming the effective projected area of the grainsdoes not exceed 10% of cathode emissive area, and that maximumpermissible leakage is to be no more than 1% of the output current perunit of area of the cathode 1, the electrical resistivity of theparticles 3 should be no less than 1 10 ohm-cm.

The compressive strength of the particles at the operating temperatureshould be greater than 150 psi. assuming a maximum of 10% coverage ofthe emissive surface of the cathode 1. A greater degree of particlecoverage may be used, but will decrease the efiiciency of the device.The compressive strength requirements will depend upon the totalexternal load to which the particles 3 are subjected and is a function,also of particle distribution density on the cathode 1. Approximaterequirements are as follows:

When the external load increases, the compressive strength of theparticles 3 must be higher, by the proportion factor of times greatercompressive strength per p.s.i. increase, in the situation of 1%particle density on the cathode surface. Where particle density is 10%of the cathode surface, then compressive strength of the particles 3must be increased 10 times for each additional p.s.i. of external load.

The particles 3 may be any of the following compounds: MgO; CaO-MgO-SiOMgO-SiO (steatite); 2Mg0 SiO (bosterite); ZrO Si0 (zircon); MgO 2Al O5SiO (cordierite); crystal forms of Al O including alumina or Alundum;ruby; corundum; sapphire; diamond; mixtures of compounds which containrelatively large amounts of A1 0 including mullite, sillimanite andfirebrick (53% SiO 43% A1 0 glass, including fused quartz, Vycor,porcelain, agate and amethyst; zirconium compounds, including ZrN, Z1'BrZrO (white) and baddeleyite (yellow or brown ZrO and the like.

The particles 3 have a size in the range of 0.2 to 1000 microns andserve to space apart the cathode 1 and the anode 2 at a fixed distance.The cathode 1 and the anode 2 may be very thin metal plates, because theparticles 3 also provide internal mechanical support in the chamberformed by the cathode 1 and the anode 2, preventing buckling due toexternal pressure and thermal expansion.

The cathode 1 and anode 2 are connected in an electrical circuit with aresistance load, not shown, by conductor leads 5 and 6, which areconnected to the cathode and anode, respectively. The resistance loadmay be an electric motor, a transistorized television receiver (whichoperates on direct current), storage batteries for domestic andindustrial use, or any other electrically driven appliance or device.

Inherently, each cell of a thermionic device is a high current, lowvoltage source of electric power. Individual thermionic cells may beelectrically connected in series or parallel to produce voltagecurrentcombinations for specific uses. Auxiliary equipment such as rotaryinverters, vibrators and transformer-rectifier combinations may be usedto produce voltage or current combinations of AC, or high voltage DC.

A seal 7 is located between the periphery of the cathode 1 and the anodeand around the conductor lead 6 where it leaves the cooling jacket 9.The seal 7 is an insulating ceramic material which has very low thermalconductivity and high electrical resistivity. Many of the materialsdescribed above for use as particles 3 would also be suitable for use asthe seal 7. Specific materials which may be used for the seal 7 arealuminum oxide, fused silica or quartz. The cathode 1 and the anode 2are bonded or fused to the seal 7, so that the chamber therebetween maybe evacuated. The seal 7 electrically separates the cathode 1 and theanode 2, and serves to complete the walls of the sealed chamber of thethermionic device. The seal 7 on the conductor lead 6 serves the samepurpose also.

Energy is applied to the device by a heat source 8 which is located withits focal point on the outer surface of the cathode 1. The heat source8, as shown in FIG- URE 1, is a gas burner, but the heat source may alsotake the form of heat of combustion, focused solar heat, heat of athermonuclear reaction, or chemical heat other than combustion. The heatsource may attain temperatures in the range from l000 K. to 4000 K. Thenormal range of the combustion heat source 8 shown in FIGURE 1 is 1000K. to 2000 K., but solar heat sources operate at temperatures closer to4000 K.

A cooling jacket 9 surrounds the outer surface of the anode 2 and servesto increase the temperature differential between the cathode and anode.The jacket 9 is connected to the seal '7 or anode 2 by brazing or fusionwelding and completely encloses the outer surface of the anode 2. Thecooling jacket 9 may be formed of any metallic material which readilyconducts away heat and defines a chamber which contains a coolingmedium. Circulating means may be provided for the liquid or gas coolantwith an inlet 12 and an outlet 13 on the cooling jacket 9. To increasethe heat transfer, the cooling jacket 9 may be equipped with coolingfins 11. The cooling fins 11 increase the total heat transfer surface,and further aid in cooling the anode 2. Alternately, in a modified formof this device which does not have a cooling jacket, the cooling fins 11may be fitted directly to the back of the anode 2. The cooling medium inchamber 10 serves to increase the temperature differential between thecathode 1 and the anode 2, thereby increasing thermionic efficiency.

The thermionic device shown in FIGURES 1-3 may be assembled in thefollowing manner. The cathode 1 is first coated on one surface with athin film of the B210 -SrO coating 4. The particles 3 may then beapplied to the coated surface of the cathode 1 by spraying, sputtering,brushing or vacuum deposition.

The anode 2, which complements the cathode 1 in size and shape, is thenplaced over the coated surface in substantial alignment therewith andpressure is applied from outside both the anode 2 and the cathode 1,forcing their opposing surfaces into close proximity in spacedrelationship, and causing the edges of the particles 3 to embed in theopposing surfaces of the anode and the cathode. As shown in FIG. 3, thesurface of the particles is irregular and the edges of the particles 3penetrate through the thin film coating 4 on the cathode 1. A resilientclamping means, such as a rubber cushioned vise, is used to force theanode and cathode together.

The seal 7 is then inserted between the peripheral edges of the anodeand cathode, electrically separating these elements and forming achamber 10a therebetween. A small opening is left in the seal 7 forevacuating the chamber 10a. The ceramic-to-metal seal may beaccomplished by a brazing method such as described in US. Patent2,836,885 to MacDonald.

Conductor leads 5 and 6 are then connected to the cathode 1 and theanode 2, respectively, by such means as soldering, fusion welding orminute bolts. The chamber 10a is then evacuated by means such as a highvacuum pump through the opening left for this purpose in the seal 7.

Evacuation of the chamber 10a causes the cathode 1 and the anode 2 tofurther bear against the particles 3, permanently disposing theparticles in fixed positions between the opposing surfaces of thecathode 1 and the anode 2. The small opening in the seal 7 is thenpermanently sealed and the structure is removed from the resilientclamping means.

The cooling jacket 9 having a small diameter opening to allow for thepassage of the conductor lead 6 from the cooled anode 2 is then disposedaround anode 2. The conductor lead 6 is passed through the opening anddrawn up tight and the peripheral edges of the cooling jacket areconnected to the seal 7 by means such as the brazing method previouslycited.

In some cases, the chamber 10a is filled with an inert gas or aplurality of ionizable gases. Such gases generally have low vaporpressure, and an external insulated clamping means may be added to holdthe cathode 1 and the anode 2 in close spaced relationship.

In operation, thermal energy is applied from the heat source 8 to thecathode 1, bringing its temperature up to the operating range, 1000 K.to 4000 K. The heated cathode 1 emits electrons from its upper surface,aided by the effect of the coating 4. Emitted electrons are collected bythe anode 2, and an builds up across the chamber 10a. Electric energyflows from the anode 2 through the conductor lead 6 to the resistanceload which uses the generated electricity to function. As previouslymentioned, a thermionic device has no moving parts, and is inherently asource of DC. electricity having low voltage and high amperage. Theclosely spaced relationship of the opposing surfaces of the anode 2 andthe cathode 1 improves the thermionic efiiciency of this device byreducing the major obstacles to high density electron flow.

Another form of a thermionic device is shown in FIG. 4. A cathode plate14, an anode-cathode plate 15, and an anode plate 16 are placed inthermal cascade to increase efficiency by utilizing excess thermalenergy, which passes through the cathode plate 14 to heat theanodecathode plate 15. The plates 14, 15 and 16 are spaced apart by aplurality of particles 17, similar in composition and function to theparticles 3 of the first embodiment. As shown in FIG. 5, the cascadeddevice is preferably circular, and the succeeding plates areprogressively larger in surface area, as shown in FIG. 4, to maintainuniform current output per cell. The combination anode-cathode plate 1.5serves a dual function in operation for its lower surface collectselectrons emitted from the cathode plate 14, and its upper surface emitselectrons which are collected by the anode plate 1.6. A seal 18 similarto the seal '7 in material and function separates and seals the edges ofthe plates and conductor leads 19 and 20 are connected only to thecathode plate 14 and the anode plate 16, respectively, and function asdo the conductor leads 5 and 6 of the first embodiment. Assembly of thisdevice is substantially as described above in connection with FIGURE 1,except the particles 17 are applied to both sides of the anode-cathodeplate 15, before assembling the anode plate 16 and the cathode plate 14on the opposite surfaces of the anodecathode plate 15.

FIG. 6 shows a second modified form of the invention in which thethermionic device has a radioactive cathode. A cylindrical radioactivecathode 19 is employed and is formed of a substance such as an uraniumcompound which readily generates heat when bombarded by neutrons, andemits electrons at a high rate. A cylindrical anode 20 is spacedoutwardly of the radioactive cathode 19 and is formed of a metal, suchas aluminum, which freely allows passage of neutrons through to thecathode 19. The anode 20 functions in a manner similar to the anode 2 ofthe first embodiment. A plurality of particles 21 in the size range from.2 to 1000 microns and substantially similar to the particles 3, aredistributed between the opposing surfaces of the cathode 19 and theanode 20. A cylindrical jacket 22 surrounds the device, and is formed ofan insulating material. A series of supporting members 23 position thecooling jacket 22 around the anode 20 and are fabricated from a materialsimilar to that of cooling jacket 22. An oil coolant is introduced intothe chamber 24- between jacket 22 and 7? anode 20 through an inlet andis withdrawn through outlet 26 during operation.

Conductor leads 27 and 28 are connected to the cath ode 19and the anode20, respectively. The conductor leads 27 and 28 are insulated from thecooling jacket 22 by means of seals 29, which are substantially similarto the seal 7 of the first embodiment.

In operation, the thermionic device shown in FIG. 6 is placed in thecore of an atomic reactor. Neutrons from the reactor bombard theradioactive cathode 19, passing through the cooling jacket 22 and theanode 28, which are both neutron windows. Neutron bombardment causes thecathode 19 to heat up and emit electrons, which are then collected bythe closely spaced anode 211. This device effectively bypasses theintermediate step performed by turbines in the present conversion ofnuclear power to electricity. It also eliminates the shielding problemsinherent in turbines which use radioactive steam. This device also hasthe advantages described above in connection with the first embodiment,that is, it has no moving parts and inherently produces low volt-- age,high amperage current.

Various modes of carrying out the invention are contemplated as withinthe scope of the following claims particularly pointing out anddistinctly claiming the subject matter which is regarded as theinvention.

I claim:

1. A thermionic converter, comprising an electron emitting cathode, anelectron collecting anode disposed in spaced relation to said cathode,means for heating the cathode, a plurality of finely divided particlesdisposed between the opposing surfaces of the anode and cathode, saidparticles being haphazardly arranged between said opposing surfaces andhaving irregular edges penetrating and imbedded in conforming irregularopenings in said surfaces to provide a substantially uniform spacing ina range from 0.2 to 1000 microns between said opposing surfaces, saidparticles having a melting point in 8 the range of 1000" K. to 4000 K.and having a high resistance to crushing and having electricalresistivity sufiiciently high to maintain electrical current lossesthrough said particles at a minimum, and means for electricallyconnecting said anode and cathode in an operating circuit whereby theelectrical energy generated by thermionic emission is utilized.

2. In a thermionic converter, comprising an electron emitting cathode,an electron collecting anode, heating means for heating the electronemitting cathode, cooling means for cooling the electron collectinganode to maintain a temperature differential between said anode andcathode, spacing means disposed between and in contact with the opposingsurfaces of the cathode and the anode, said spacing means comprisingfinely divided particles of a substance selected from the group ofmetallic oxides consisting of A1 0 ZrO Si0 MgO, BeO and mixtures thereofand having a size in the range of 0.2 to 10 microns, said particlesbeing randomly distributed between the opposing surfaces, being ofirregular shape and having jagged edges penetrating and irnbedded in theopposing surfaces of said anode and cathode, whereby the thermalefiiciency of the thermionic converter is substantially increased bydecreasing the electric potential barriers to electron flow from theelectron emitting cathode to the electron collecting anode.

References Cited in the file of this patent UNITED sTATEs PATENTS264,953 Edison Sept. 19, 1882 2,661,431 Linder Dec. 1, 1953 2,686,958Eber Aug. 24, 1954 2,881,384 Durant Apr. 7, 1959 2,887,606 Diemer et al.May 19, 1959 2,899,590 Sorg Aug. 11, 1959 2,916,649 Levin Dec. 8, 19592,919,356 Fry Dec. 29, 1959

1. A THERMIONIC CONVERTER, COMPRISING AN ELECTRON EMITTING CATHODE, ANELECTRON COLLECTING ANODE DISPOSED IN SPACED RELATION TO SAID CATHODE,MEANS FOR HEATING THE CATHODE, A PLURALITY OF FINELY DIVIDED PARTICLESDISPOSED BETWEEN THE OPPOSING SURFACES OF THE ANODE AND CATHODE, SAIDPARTICLES BEING HAPHAZARDLY ARRANGED BETWEEN SAID OPPOSING SURFACES ANDHAVING IRREGULAR EDGES PENETRATING AND IMBEDDED IN CONFORMING IRREGULAROPENINGS IN SAID SURFACES TO PROVIDE A SUBSTANTIALLY UNIFORM SPACING INA RANGE FROM 0.2 TO 1000 MICRONS BETWEEN SAID OPPOSING SURFACES, SAIDPARTICLES HAVING A MELTING POINT IN THE RANGE OF 1000* K. TO 4000* K.AND HAVING A HIGH RESISTANCE TO CRUSHING AND HAVING ELECTRICALRESISTIVITY SUFFICIENTLY HIGH TO MAINTAIN ELECTRICAL CURRENT LOSSESTHROUGH SAID PARTICLES AT A MINIMUM, AND MEANS FOR ELECTRICALLYCONNECTING SAID ANODE AND CATHODE IN AN OPERATING CIRCUIT WHEREBY THEELECTRICAL ENERGY GENERATED BY THERMIONIC EMISSION IS UTILIZED.